1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method that process multi-valued image data that corresponds to the same area in order to print images in the same area by relatively moving a print unit a plurality of times or by relatively moving a plurality of printing element groups with respect to the same area of a printing medium.
2. Description of the Related Art
An inkjet printing method that prints dots on a printing medium by ejecting ink from printing elements (nozzles) is known as an example of a printing method that uses a printing head comprising a plurality of printing elements for printing dots. This kind of inkjet printing apparatus can be categorized as full-line type or serial type according to differences in construction. Regardless of whether the device is full-line type or serial type, variation occurs in the ejection volume and ejecting direction of the plurality of printing elements of the printing head. In addition, due to these kinds of variations, density unevenness or stripes may occur in the image.
A multi-pass printing method is known as technology for reducing this kind of density unevenness or stripes. In the multi-pass printing method, image data that is to be printed on the same area of a printing medium is divided into image data to be printed in a plurality of printing scans. Moreover, the divided image data is sequentially printed according to the plurality of printing scans with a conveyance operation of the printing medium in between each scan. By doing so, even though there are variations in ejection characteristics of each of the individual print elements, the dots printed by one print element does not continuous in the scanning direction and the characteristics of individual print elements are dispersed over a wide area. As a result, it is possible to obtain a uniform and smooth image.
This kind of multi-pass printing method can be applied to either a serial type or full-line type printing device that comprise a plurality of printing heads (or a plurality of printing element groups) that eject the same kind of ink. That is, image data is divided for a plurality of printing element groups that eject the same kind of ink, and that divided image data is printed during at least one relative movement for each of the plurality of printing element groups. As a result, even though there is variation in the ejection characteristics of the individual print elements, it is possible to reduce the effect of that variation. Furthermore, it is possible to combine the two printing methods described above, and to print an image by performing printing scanning a plurality of times while using a plurality of printing element groups that eject the same kind of ink.
Conventionally, in performing this kind of division of image data, masks were used for which data that allows printing of dots (1: data that does not mask image data) and data that does not allow printing of dots (0: data that masks image data) are arranged beforehand. More specifically, by performing a logical AND operation between binary image data to be printed on the same area of a printing medium and the aforementioned mask, the binary image data is divided into binary image data that is to be printed by each printing scan or each printing head.
In this kind of mask, the arrangement of data that allows printing (1) is set so that there is a complementary relationship between the plurality of printing scans (or plurality of printing heads). In other words, pixels that are set to be printed (1) in the binarized image data are such that one dot is printed by either one printing scan or one printing head. By doing so, the image information before division is saved even after division.
However, recently, by performing the multi-pass printing described above, a new problem has emerged in that changes in density or density unevenness occur due to printing position displacement (registration) in units of printing scans or printing heads (printing element groups). The printing position displacement in printing scan units or printing element group units referred to here is as described below. That is, this printing position displacement is a shift between dot groups (planes) such as a shift in the dot group (plane) that is printed by the first printing scan (or a printing element group) and the dot group (plane) that is printed by the second printing scan (or a different printing element group). The shift between these planes is caused by fluctuation in the distance between the printing medium and the ejection port face and fluctuation in the amount the printing medium is conveyed. In addition, when shifting does occur between planes, there is fluctuation in the dot coverage, which causes fluctuation in image density or image unevenness. As was described above, dot groups and pixel groups that are printed by the same printing scan of the same unit (for example, one printing element group that ejects the same kind of ink) are hereafter called a ‘plane’.
As described above, recently there is a demand for even higher quality images, and a data image processing method during multi-pass printing that is capable of preventing a shift in the printing position between planes that is caused by fluctuation of various printing conditions is desired. Hereafter, in this specification, resistance to density fluctuation or density unevenness that is caused by shifting in the printing position between planes due to any printing condition is referred to as ‘robustness’.
Japanese Patent Laid-Open No. 2000-103088 and Japanese Patent Laid-Open No. 2001-150700 disclose an image data processing method for improving robustness. According to these disclosures, it is focused on that the fluctuation of image density due to fluctuation of various printing conditions is caused by a perfect complementary relationship with the binary image data after being distributed so that the image data corresponds to different printing scans or different printing element groups. Moreover, these disclosures point out that by creating image data that corresponds to different printing scans or different printing element groups such that the complementary relationship is reduced, it is possible to achieve multi-pass printing with excellent ‘robustness’. Furthermore, in these disclosures, in order that large density fluctuation does not occur even when there is shifting between a plurality of planes, multi-valued image data before binarization is divided corresponding to different printing scans or printing element groups, and that divided multi-valued image data is then binarized independently (without correlation).
FIG. 10 is a block diagram for explaining the image data processing method that is disclosed in Japanese Patent Laid-Open No. 2000-103088 or Japanese Patent Laid-Open No. 2001-150700. Here, the case is illustrated in which multi-valued image data is divided between two printing scans. The multi-valued image data (RGB) that is inputted from a host computer is converted by a palette conversion process 12 to multi-value density data (CMYK) that corresponds to the ink color of the printing apparatus. After that, the multi-value density data (CMYK) undergoes gradation correction by a gradation correction process. The following processing is performed independently for each of the colors black (K), cyan (C), magenta (M) and yellow (Y).
The multi-value density data of each color is distributed by an image data distributing process 14 to first scan multi-value data 15-1 and second scan multi-value data 15-2. In other words, when the number of multi-valued image data for black is ‘200’, for example, the image data is distributed into ‘100’, which corresponds to half of ‘200’, for the first scan, and similarly, is distributed into ‘100’ for the second scan. After that, the multi-value data 15-1 for the first scan undergoes quantization processing by a first quantization process 16-1 according to a specified diffusion matrix, then is converted to binary data 17-1 for the first scan and stored in a band memory for the first scan. On the other hand, the multi-value data 15-2 for the second scan undergoes quantization processing by a second quantization process 16-2 according to a diffusion matrix different from that of the first quantization process then is converted to binary data 17-2 for the second scan and stored in a band memory for the second scan. In the first printing scan and second printing scan, ink is ejected according to the binary data that is stored in the respective band memory. In FIG. 10, the case of distributing the data for one image between two printing scans was explained; however, in Japanese Patent Laid-Open No. 2000-103088 and Japanese Patent Laid-Open No. 2001-150700, the case in which the data for one image is distributed between two printing heads (two printing element groups) is also disclosed.
FIG. 14A is a diagram that illustrates the arrangement state of dots (black dots) 1401 to be printed in a first printing scan and dots (white dots) 1402 to be printed in a second printing scan when dividing image data using a mask pattern having a complementary relationship. Here, the case is illustrated in which 255 density data are inputted for all of the pixels, and one dot is printed for all of the pixels by either the first printing scan or second printing scan. In other words, the dots that are printed by the first printing scan and the dots that are printed by the second printing scan are arranged so that they do not overlap each other.
On the other hand, FIG. 14B is a diagram that illustrates the arrangement state of dots when distributing the image data according to the method disclosed in Japanese Patent Laid-Open No. 2000-103088 and Japanese Patent Laid-Open No. 2001-150700. In the diagram, the black dots are dots 1501 that are to be printed in a first printing scan, the white dots are dots 1502 that are to be printed in a second printing scan, and the gray dots are dots 1503 that are printed by overlapping of the first printing scan and second printing scan. In FIG. 14B, there is no complementary relationship between the dots to be printed by the first printing scan and the dots to be printed by the second printing scan. Therefore, when compared with the case in FIG. 14A where the dots are in a complete complementary relationship, portions (gray dots) 1503 occur where two dots overlap, and there are blank areas where no dots are printed.
Here, the case in which a first plane, which is a collection of dots to be printed by the first printing scan, and a second plane, which is a collection of dots to be printed in the second printing scan, are shifted by the amount of one pixel in either the main scanning direction or the sub-scanning direction is feasible. In that case, when the first plane and second plane are in a complete complementary relationship as in FIG. 14A, the dots that are printed in the first plane completely overlap the dots that are printed in the second plane, so areas of blank paper are exposed, and there is a large drop in image density. Even when there is shifting that is not as large as one pixel, fluctuation of distance between or overlapping of adjacent dots greatly affects the coverage and image density of dots in blank areas. That is, it is known that when this kind of shifting between planes changes according to fluctuation in the distance between the printing medium and the ejection port face (distance to the paper), or fluctuation in the amount the printing medium is conveyed, that the image density will also fluctuate, causing density unevenness.
On the other hand, in the case of FIG. 14B, even when there is shifting as much as the amount of one pixel between the first plane and second plane, there is not much fluctuation in coverage of dots on the printing medium. Areas where there is overlap of the dots that are printed in the first printing scan and the dots that are printed in the second printing scan newly appear; however, there are also areas where two dots that were already overlapping are separated. Therefore, when making a judgment over a large area, there is not much fluctuation of the coverage of the dots on the printing medium, so it is also difficult for fluctuation in image density to occur. In other words, by employing the method disclosed in Japanese Patent Laid-Open No. 2000-103088 or Japanese Patent Laid-Open No. 2001-150700, even though there is fluctuation in the distance between the printing medium and the ejection port face (distance to the paper) or fluctuation in the amount the printing medium is conveyed, it is possible to suppress the fluctuation in image density or density unevenness that is caused by these, and thus it is possible to output an image having excellent robustness.
However, in the method disclosed in Japanese patent laid-open No. 2000-103088 or Japanese Patent Laid-Open No. 2001-150700, the plurality of planes are not correlated with the binary data, so there may be cases in which graininess becomes worse. For example, as illustrated in FIG. 15A, when taken from the aspect of reducing graininess, in highlighted portions, the ideal is to disperse dots evenly maintaining a predetermined distance between the small number of dots (1701, 1702). However, as illustrated in FIG. 15B, in construction in which the plurality of planes are not correlated with the binary data, locations (1603) where dots overlap, and locations (1601, 1602) where dots are printed touch each other occur irregularly, this clump of dots causes the graininess to become worse. It is not shown in the figure; however, when irregular dot overlap occurs in high-density areas, the dot coverage on the paper surface drops, causing a decrease in density.